This disclosure concerns a new selective catalytic reduction process for the conversion of nitrogen oxides (NO.sub.x) in the presence of oxygen in exhaust gases from stationary or moving sources (power plants, internal combustion engines (compression or spark ignited), lean-burn engines, industrial processes, etc.) to harmless gases.
Much research effort has been expended in attempting to remove, or at least materially reduce, the NO.sub.x content of effluent gases, particularly exhaust gases from internal combustion engines, e.g., automobile engines. High compression engines, especially diesel engines, produce unacceptably high concentrations of NO.sub.x in the exhaust, and environmental considerations have mandated the removal of such oxides of nitrogen to an acceptable level.
Catalytic treatment of fluids containing NO.sub.x to lower the content of these oxides in the effluent gas is not new per se. For a substantial treatise on the subject, reference may be had to the book "Nitrogen Oxides, Control and Removal-Recent Developments" by L. H. Yaverbaum, Noyes Data Corp., 1979 which summarizes many of the recently issued U.S. Patents in this field. Catalytic converters for use in the exhaust lines of internal combustion engines are now commonplace. Most usually these consist of a honeycomb of ceramic, or corrugated thin metal strips fan-folded or spirally wrapped, and having a catalyst or catalysts deposited on the surface or surfaces of the supporting device. The exhaust gas is conducted through the "honeycomb" where contact with the catalyst or catalysts is effected and various pollutants chemically or physically altered to environmentally acceptable components before discharge into the atmosphere.
Several technical solutions have been proposed to reduce NO.sub.x emissions from stationary sources with oxygen present, and many have been put into operation. These include combustion modifications, gas scrubbing, noncatalytic reduction, nonselective catalytic reduction, and selective catalytic reduction (SCR).
SCR processes have been documented in literature and patents for several years. These involve mixing ammonia gas with hot exhaust gases prior to flow through a catalytic packed bed or monolith converter wherein an overall reaction such as (1) is carried out at better than 80% conversion: EQU 4NH.sub.3 +6NO.fwdarw.5N.sub.2 +6H.sub.2 O (1)
Reaction (1) is not exact, nor is it representative of a specific single surface reaction. Other products such as N.sub.2 O and NO.sub.2 may be made.
There is no universal agreement about the fundamental reaction steps in the SCR, i.e., the reaction mechanism, and it may differ depending on catalyst type and temperature as well as gas composition. However, evidence seems to support a two-step process where reactions (2) and (3) occur in parallel. EQU NO+1/2O.sub.2 .fwdarw.NO.sub.2 ( 2) EQU 6NO.sub.2 +8NH.sub.3 .fwdarw.7N.sub.2 +12H.sub.2 O (3)
Various authors have noted that the reduction of NO requires the presence of oxygen and that the reduction of NO.sub.2 does not, and the latter is a much easier reaction to carry out. It is believed that reaction (2) is the rate limiting step. However, all SCR processes disclosed so far involve passing the ammonia/exhaust gas mixture over a catalyst bed such that the above reactions must occur in parallel on the same catalyst under the same operating conditions in order to achieve overall reaction (1).
Many catalysts have been disclosed for SCR processes. One useful classification is between noble and base metals. SCR processes with noble metals (Pt, mainly) can run at lower temperatures and higher space velocities than those using base metals. Noble metal processes, typically run in the 180.degree.-250.degree. C. range, are limited on the low end by a tendency to form the potentially explosive ammonium nitrate and on the high end by a tendency to oxidize ammonia back to NO.sub.x. Base metal processes, typically run at 300.degree.-450.degree. C., are limited by rate on the low end and ammonia oxidation on the high end.
The thermodynamic equilibrium of reaction (2), believed to be the rate-limiting step, has an important effect on the reaction rate. Equilibrium curves for various oxygen levels at one atmosphere are shown in FIG. 2. These show that in the temperature range of existing SCR processes, there is a significant limitation on the production of NO.sub.2 from NO at any point in the reactor.
The present disclosure takes these facts into account in outlining an SCR process that is more economical than those heretofore disclosed.
Reference may be had to the patent to Tadokoro et al U.S. Pat. No. 4,278,639 for one form of apparatus in which the process of this invention may be carried out. This reference discloses a catalytic converter which comprises a casing having an inlet and an outlet, and at least two separate catalyst carriers within the casing. Different catalysts may be provided in the respective catalyst carriers, preferably a reduction catalyst in the first carrier and an oxidation catalyst in the second. A spacer ring is provided for connecting the catalyst carriers together in longitudinally spaced and aligned relation to each other to provide a single unitary structure of catalyst carriers. A cushioning layer is provided between the unitary structure and the casing. Other U.S. patents of interest in this field are the patents to Retallick 4,301,039; 4,402,871; 4,597,262 and 4,576,800.
The disclosure of the above-mentioned Tadokoro et al patent is incorporated herein by reference.
The use of a plurality of catalysts for treating a fluid stream is known. These may be mixed as in a platinum/rhodium catalyst and applied to a carrier, or they may be separately applied to sequentially disposed carriers as shown in the apparatus of Tadokoro et al supra. The introduction of gas or vapor between sequential catalyst carriers is also known (see Tadokoro et al, supra.).
The improvement in the present invention over the prior art is in the spatial arrangement of different catalysts on sequentially located catalyst carriers and the introduction of ammonia between the catalyst carriers. The first catalyst carrier is provided with an oxidizing catalyst and the second with a catalyst for the reduction of NO.sub.2 by NH.sub.3. Although such catalysts have previously been used to reduce NO.sub.x in a fluid stream, where ammonia has been used in the system, the ammonia has always been introduced before the fluid inlet into the catalytic converter. The catalyst beds or carriers may be in the same envelope as shown in Tadokoro (supra). or in separate, albeit sequential, envelopes in the same fluid conduit.
This process for treating gas or fluid containing NO.sub.x and O.sub.2 with NH.sub.3 to obtain principally N.sub.2 and H.sub.2 O has the following features:
1. Two catalytic reactors or beds in series, one for the oxidation of NO to NO.sub.2 in high yield without ammonia present, and the other for the reduction of NO.sub.2 with ammonia. Ammonia is injected before the second reactor.
2. Optimization of temperature for each stage. The temperature of the first bed must be low enough for an acceptably high conversion of NO to NO.sub.2. Otherwise, the two bed temperatures are conveniently selected on an economic basis to maximize overall NO.sub.x conversion while minimizing side reactions such as the formation of ammonium nitrate or sulfate compounds.
3. Optimization of catalyst for each stage, e.g., noble metal for oxidation, base metal for reduction. The process can utilize whatever catalysts work best. It is not essential that the oxidizing catalyst be from the platinum group nor that the reducing catalyst be from the base metal group although these are preferred.
The net effect of separating the reactions is to reduce significantly the total catalyst volume required for the process, a major cost savings.
The elimination of ammonia presence during the NO oxidation step means that a low temperature consistent with high equilibrium conversion of NO to NO.sub.2 may be used, e.g., 100.degree.-200.degree. C., without concern about ammonium nitrate formation.
The feeding of a high proportion of NO.sub.2 to NO to the SCR reactor means that a significantly lower temperature and higher space velocity may be used than with NO as the feed reactant. It is possible to consider a catalyst run in the 200.degree.-300.degree. C. range.